The following describes a new invention in the field of capacitive touch screens or 2-dimensional capacitive transducing (2DCT) sensors. U.S. Pat. No. 6,452,514, U.S. Pat. No. 7,148,704 and U.S. Pat. No. 5,730,165 disclose a capacitive measurement technique which makes it possible to create touch responsive transparent or opaque sensing regions that can detect human touch through several millimeters of plastic or glass. Described herein is a new structure for a touch screen that allows significant enhancement in both operation and appearance of the sensor.
U.S. Pat. No. 6,452,514 describes a capacitive measurement technique which is incorporated by reference herein, that uses a transmit-receive process to induce charge across the gap between an emitting electrode and a collecting electrode (the transmitter and the receiver respectively, also referred to as X and Y). The capacitive sensing described in U.S. Pat. No. 6,452,514 may be referred to as mutual capacitive or active type 2DCT sensors. As a finger touch interacts with the resulting electric field between the transmitter and receiver electrodes, the amount of charge coupled from transmitter to receiver is changed. A particular feature of the measurement technique is that most of the electric charge tends to concentrate near to sharp corners and edges (a well known effect in electrostatics). The fringing fields between transmitter and receiver electrodes dominate the charge coupling. The electrode design therefore tends to focus on the edges and the gaps between neighboring transmitter and receiver electrodes in order to maximize coupling and also to maximize the ability of a touch to interrupt the electric field between the two, hence giving the biggest relative change in measured charge. Large changes are desirable as they equate to higher resolution and equally to better signal to noise ratio.
A specially designed control chip can detect these changes in charge. It is convenient to think of these changes in charge as changes in measured coupling capacitance between transmitter and receiver electrodes (charge is rather harder to visualize). The chip processes the relative amounts of capacitive change from various places around the touch screen and uses this to compute the absolute location of touch as a set of x and y coordinates. In order for this to be possible a set of spatially distributed electrodes must be used. Commonly, these electrodes are required to be transparent so that the touch screen can operate in front of a display such as a liquid crystal display (LCD) screen or other display screen type, for example organic light emitting diode (OLED) type screens. To achieve this electrodes are often fabricated from a material known as Indium Tin Oxide (ITO) but other transparent conductive materials are also suitable. ITO has desirable properties in optical terms, but can be substantially ohmic which can have a negative impact on capacitive measurements if the resistance and capacitance combination leads to time constants that prevent timely settling of the charge transfer process.
Another example 2DCT is disclosed in US20070062739A1.
In order to create a sensor that can report the absolute coordinates of the location of the touch (or more than one touch) on the surface of the sensor or the overlying plastic or glass panel, the electrode arrangement must be specifically designed to optimize the following aspects:                accuracy of the reported touch location i.e. correspondence between real physical location and reported location. This is broadly known as “linearity” or “non-linearity” when referring to the measured error.        immunity of the sensor to external electrical noise sources.        sensitivity of the sensor to human touch i.e. its ability to detect a touch through thicker panel materials, or to detect a lighter or smaller touch.        spatial resolution of the sensor i.e. its ability to report small changes in touch location.        quality of the output in terms of the noise or jitter amplitude in the reported location.        optical quality of the sensor for the transmission of light, for factors like its transparency, its hue, its haze, the overall electrode pattern visibility etc.        optical behavior of the sensor to shallow angle reflected light i.e. the visibility of the electrode pattern and any color shifts in the reflected light.        minimizing any errors induced in the reported location caused by slight mechanical flexing during human touch. This tends to cause a change in the distance between the sensor and any underlying display or other mechanical grounded structure which in turn causes capacitive changes similar to a touch.        reducing the electrical resistance of the electrodes to allow efficient capacitive sensing within an acceptable time (often the overall measurement time of the touch screen needs to be at or below 10 ms so limiting the amount of settling time that can be used to make each measurement).        reducing the number of layers in the physical construction to minimize manufacturing cost and to improve optical properties.        reducing side-effects in the quality of reported coordinates or in the ability of the sensor to detect a touch, near to the edges of the sensor. This region typically presents difficult challenges in this regard because of the non-uniformity of the electrode pattern (its ends) and the fact that interconnecting tracks tend to reside at the edges of the sensor.        reducing the total number of electrodes used as each electrode requires some connection to the control chip and so more electrodes equates to a more complex chip and hence higher cost.        
In order to optimize linearity, the electrode pattern design is critical. Linearity is one of the primary measures of quality of a touch screen because as the linearity degrades, it becomes harder to report an accurate touch location in some regions of the screen. A sensor design that offers excellent intrinsic linearity is a key goal therefore. While it is possible to mathematically correct such non-linearity via well known techniques such as a look-up table or piecewise-linear correction, any of these methods actually trades off spatial resolution for reported linearity, and so is always a compromise.
In designing the electrodes a key objective is to arrange that the electric field that propagates from transmitter to receiver does so in a way that causes a smooth and progressive gradation from one electrode to the next. This way, as a touch moves from region to region, the capacitive change measured by the control chip also changes in a smooth and progressive way and hence contributes to good intrinsic linearity. The touch itself actually influences this process significantly and will tend to “mix” the fields from neighboring electrodes. This contributes to the overall smoothness of transition, but does tend to lead to some variation in linearity depending on the size of the touch applied. Again, electrode design needs to be carefully considered to optimize the linearity across a range of touch sizes.
As described above the quality of the output in terms of the noise or jitter amplitude in the reported location should be optimized. However, 2DCT sensors can be sensitive to external ground loading. Furthermore, electrical noise generated from LCD screens can interfere with capacitance measurements when a pointing object approaches the screen. Known methods to minimize the effects of noise on capacitive coupling is to increase the separation or air gap between an LCD screen and an overlaying 2DCT sensor. Alternatively a shielding layer may be incorporated between the LCD screen and a 2DCT sensor to reduce or block the noise induced by the LCD screen.
WO 2009/027629 published on 5 Mar. 2009 describes a capacitive touch sensor comprising a dielectric panel overlying a drive electrode with two sense electrodes. One of the sense electrodes is positioned to be shielded from the drive electrode by the first sense electrode, so that the first sense electrode receives the majority of the charge coupled from the drive electrode and the second sense electrode primarily registers noise. A sensing circuit including two detector channels is connected to the first (coupled) and second (noise) sense electrodes to receive signal samples respectively. The sensing circuit is operable to output a final signal obtained by subtracting the second signal sample from the first signal sample to cancel noise.
However, the methods described above increase the size and thickness, and may decrease the resolution of a device incorporating a display screen with a 2DCT sensor when it is more fashionable and desirable to produce smaller devices. Furthermore, additional steps are required during manufacture and as a result there is an increased cost due to further components being needed.
European patent EP 1821175 describes an alternative solution to reduce the noise collected on a 2DCT touch sensor. EP 1821175 discloses a display device with a touch sensor which is arranged so that the two dimensional touch sensor is overlaid upon a display device to form a touch sensitive display screen. The display device uses an LCD arrangement with vertical and horizontal switching of the LCD pixels. The touch sensing circuit includes a current detection circuit, a noise elimination circuit as well as a sampling circuit for each of a plurality of sensors, which are arranged to form the two-dimensional sensor array. The current detection circuit receives a strobe signal, which is generated from the horizontal and vertical switching signals of the LCD screen. The strobe signal is used to trigger a blanking of the current detection circuit during a period in which the horizontal switching voltage signal may affect the measurements performed by the detection circuit.
WO 2009/016382 published on 5 Feb. 2009 describes a sensor used to form a two dimensional touch sensor, which can be overlaid on a liquid crystal display (LCD) screen. As such, the effects of switching noise on the detection of an object caused by a common voltage signal of the LCD screen can be reduced. The sensor comprises a capacitance measurement circuit operable to measure the capacitance of the sensing element and a controller circuit to control charging cycles of the capacitance measurement circuit. The controller circuit is configured to produce charging cycles at a predetermined time and in a synchronous manner with a noise signal. For example, the charge-transfer cycles or ‘bursts’ may be performed during certain stages of the noise output signal from the display screen, i.e. at stages where noise does not significantly affect the capacitance measurements performed. Thus, the sensor can be arranged to effectively pick up the noise output from a display screen and automatically synchronize the charge-transfer bursts to occur during stages of the noise output cycle.
However, noise reduction techniques such as those described above require more complex measurement circuitry. This makes the measurement circuitry more expensive and may increase the time taken to complete an acquisition cycle.
It would therefore be desirable to provide an electrode pattern suitable for mutual capacitive or active type 2DCT sensor that can be embodied with an electrode pattern with reduced noise pick-up.